The disclosure relates generally to coating techniques and systems, and more particularly, to a wire arc spray system using a composite wire for creating a porous coating and a related method.
Metal abradable coatings include porous metal coatings and are used in a variety of industries. For example, metal abradable coatings may be applied to an inside of a casing of a gas turbine or a compressor of a jet engine to create abradable seal coatings that act on moving parts to create precise tolerances for sealing. Currently, metal abradable coatings are applied by plasma spray or combustion spray using powders that include a metal and a material that can be burned out such as a polymer (e.g., polyester) or other low melting point material. In some case, a solid lubricant such as boron nitride is also employed. The materials that are burned out are some times referred to as fugitive phase material. Once the composite materials are applied to a surface, they are heat treated to burn out the of the fugitive phase materials, resulting in a porous, abradable coating of the metal material on the surface. Another approach to creating metal abradable coatings is to bond the fugitive phase materials to the metal powder with a low temperature adhesive that will burn off during heat treatment. Abradable coatings can be expensive to create due to the processes used and non-portability of the equipment and materials used. The composite powders used in the plasma spray process also suffer from reproducibility problems and sole source limitations on the fugitive phase materials leading to expensive composite powder prices.
Wire arc spraying is another approach for coating a surface with a very dense, non-porous material. Porous metal coatings have, however, been produced from wire arc spraying by using a sacrificial metal wire, e.g., of zinc, with a dissimilar metal wire, e.g., of nickel, and electro-chemically etching out the sacrificial metal to leave porous metal behind. This application generates porous metal advantageous for, e.g., fuel cell electrodes, but not abradable seal coatings. Wires having aluminum cladding and an alumina core have also been employed for creating anti-skid coatings. In this case, the alumina does not melt (melting point 2072° C.) with the aluminum (melting point 660.32° C.) and is not removed, but is trapped in the coating, resulting in a very high surface roughness.
Other efforts to create abradable coatings generate complex honeycomb structures on the surface with various fillers in the cells.